Porous cellulose beads are safer than beads composed of a synthetic polymer, and the non-specific adsorption thereon is small. In addition, the mechanical strength of porous cellulose beads is high, and the beads have many hydroxy groups, which can be used for introducing a ligand capable of interacting with a target substance to be adsorbed. Accordingly, porous cellulose beads are used as a base material for various adsorbents such as an adsorbent for chromatography and an affinity adsorbent. Among the examples, an affinity adsorbent is used as a medical adsorbent and an adsorbent for purifying a medical antibody, since a target substance can be purified and an undesired substance amount can be reduced efficiently by using an affinity adsorbent. In particular, as a medical adsorbent for treating rheumatism, hemophilia or dilated cardiomyopathy, an adsorbent produced by immobilizing Protein A as an affinity ligand on a porous carrier has attracted attention (for example, Non-patent document 1 and Non-patent document 2).
In addition, it has attracted attention that an adsorbent produced by immobilizing Protein A as an affinity ligand on a porous carrier is used as an adsorbent for purifying an antibody pharmaceutical by specifically adsorbing an immune globulin, i.e. IgG.
Many methods for producing porous cellulose beads require a cumbersome step in comparison with the case of using a general synthetic polymer, since cellulose is considered to be hardly dissolved. As such a method, for example, Patent document 1 discloses a method in which cellulose is dissolved in a solvent such as calcium thiocyanate aqueous solution and coagulated. Such a solvent is highly corrosive and toxic, and it is difficult due to the solvent to design a plant. The cellulose solution used in the method exhibits peculiar behaviors, and the porous cellulose beads obtained by the method have considerably large pores and broad pore size distribution (for example, Non-patent document 3). When such porous cellulose beads obtained by the method are used for an adsorbent to adsorb an antibody and the like, high adsorption performance cannot be expected, since the specific surface area thereof is small. In addition, for example, Patent Document 2 discloses a method for producing a porous cellulose carrier by binding a substituent group to the hydroxy group of cellulose in order to improve the solubility of the cellulose, dissolving the cellulose in a general solvent to carry out agglomeration, and then removing the substituent group. However, the steps of the method are cumbersome and molecular weight may be decreased during the steps of reacting and removing the substituent group. Thus, the strength of the carrier tends to be not enough to be used in high-speed processing and large scale which have been recently required.
Furthermore, for example, Patent documents 3 and 4 disclose a method in which cellulose is dissolved in sodium hydroxide aqueous solution having low temperature. However, in the method described in Patent document 3, after the step of heating a mixture of cellulose and a hydrogen bond-cleaving solution at 100 to 350° C. under pressure, the mixture is dissolved in an alkaline aqueous solution. Such a step is industrially disadvantageous. In addition, the method described in Patent document 4 requires the steps in which cellulose is dispersed in a strong base solution, and the dispersion is once freezed and then melted.
Patent document 5 discloses the cellulose which can be dissolved in an alkaline solution. However, the cellulose is a micro fiber having a diameter of 1 μm or less, and further micronized to 500 nm or smaller. Such a micronizing procedure is not suitable for industrial production.
Very recently, Patent document 6 discloses a method in which microbial cellulose is dissolved in an alkaline solution to prepare a cellulose solution, the cellulose solution is particulated by adding a dispersion medium, and the microbial cellulose particle is freezed and then washed to obtain cellulose beads. However, the method is not suitable for an industrial production due to the cumbersome steps.